The importance of gene duplication in evolution has long been recognized. Because duplicated genes are prone to diverge in function, gene duplication could plausibly play a role in species differentiation. However, experimental evidence linking gene duplication with speciation is scarce. Here, we show that a hybrid-male sterility gene, Odysseus (OdsH), arose by gene duplication in the Drosophila genome. OdsH has evolved at a very high rate, whereas its most immediate paralog, unc-4, is nearly identical among species in the Drosophila melanogaster subgroup. The disparity in their sequence evolution is echoed by the divergence in their expression patterns in both soma and reproductive tissues. We suggest that duplicated genes that have yet to evolve a stable function at the time of speciation may be candidates for "speciation genes," which is broadly defined as genes that contribute to differential adaptation between species.